In a known device of that type, for example as described in document FR 2 386 359, the bottom of the crucible includes a narrow slot through which the support, which is a carbon ribbon, penetrates into the bath and passes vertically therethrough, traveling in an upward direction.
Nevertheless, it is found that the thickness of the silicon layer deposited on the faces of the ribbon decreases rapidly on approaching the two side edges of the ribbon. This results in the deposited layer of silicon presenting two lateral margins along each of the two side edges with physical characteristics that are degraded relative to those in the middle portion of the layer. These side margins are practically unusable, in particular for making photovoltaic cells, and they constitute sources of fractures occurring in the silicon layers.
On the edges of the carbon ribbon, the liquid silicon meniscus changes from a two-dimensional shape with a solid-liquid connection line situated about 6.8 millimeters (mm) above the bath, to a shape that is circularly symmetrical. Under such circumstances, the solid-liquid connection line, which to the first order is a function of the thickness of the carbon ribbon in the absence of any particular control means, becomes situated at a height that is very low above the bath. In the “transition” zone up to the edge of the ribbon, i.e. over a width of about 5 mm, the solid-liquid connection line moves down from 6.8 mm to a value that is typically 1 mm to 2 mm. In this zone, the thickness of the deposit decreases rapidly. The deposit becomes fragile and its crystal quality is degraded (grains of small size).
In an attempt to control the edges, a device is disclosed in document FR 2 550 965 that further includes two chutes of semicircular section that are held vertically respectively on either side of the side edges of the ribbon passing through the bath. The concave sides of the chutes face towards the respective side edges of the ribbon, with the chutes being dipped in part in the bath so as to raise the level of the bath by capillarity in the vicinity of the side edges of the ribbon.
In order to control the capillary rise of liquid silicon along the chutes, the following parameters should be varied: the inside diameter of the chutes, and the distance and orientation of the chutes relative to the edges of the ribbon.
That device does indeed make it possible to make deposits of silicon on a carbon ribbon with an increase in the useful area deposited, presenting fewer fractures, and with the deposited silicon layers presenting improved electrical properties.
Nevertheless, that device is still not optimized. The raising of the solid-liquid connection line at the edges remains small, and the transition zone is still too wide. In addition, the reduction in the thickness in the margins remains large and this zone continues to be a source for generating fractures during cooling and subsequent handling of the ribbon.